Compute starting seconds as hours * 3600 + minutes * 60 + seconds. Compute ending seconds as hours * 3600 + minutes * 60 + seconds. The difference in seconds can be converted back to hours / minutes / seconds easily enough.
Things to consider:
1) will your data span a day boundary (start at 11 PM, end at 1 AM)?
2) is your data in 12-hour or 24-hour format?
3) can your data span more than one day? If so, how do you detect that?
4) is the data computer-generated, or human-generated? If the latter, do you need to think about one-digit versus 2-digit hours, minutes, seconds?
5) do you need to be concerned with daylight savings time? If so, how do you detect that?
That should start you thinking in the right direction.